When pilots, first officers and second officers submit their requests for their following month’s schedule, they also submit the names of any coworkers who stress them out and who, if they were put in the flight deck together, could compromise the safety and security of the plane.

This list is then fed into the airline’s scheduling system, which will keep those two people from working on a flight together.

The “do not pair” list is how airlines ensure that the flight deck remains a calm, communicative environment and a place where, if necessary, pilots and captains can perform tricky maneuvers without any added stress.

However the system is in place for more than just social niceties. In a story for the Los Angeles Times, pilot Elliott Hester recounted a tale about a captain whose personality was so “corrosive,” his second officer punched him in the face before storming off the plane. Because the flight could not leave without a second officer, more than 200 passengers were delayed until tempers cooled.